So what the heck is up with all those pine-needles? The pine-needles are probably the most noticeable recurrent image in the book. You get them in the very first sentence:
He lay flat on the brown, pine-needled floor of the forest, his chin on his folded arms, and high overhead the wind blew in the tops of the pine trees. (1.1)
And the very last sentence:
He could feel his heart beating against the pine-needle floor of the forest. (43.402)
You also get frequent mention of them whenever Robert Jordan is in his sleeping robe at night (usually before Maria comes to join him), and occasionally at other times too, as when he feels them under his feet as he walks to the bridge on the morning of the mission (41.79).
It's really up to you how much you want to read into the pine needles. Regardless of whether they have any larger meaning, Hemingway's use of the same image at the beginning and end of the book – not just pine needles, but Robert Jordan lying on them – gives the novel nice bookends, and a nifty little sense of circularity.
Reading a little more into it, it's likely that the pine needles on the ground are meant to be the singular image for the land of Spain itself, for Spanish earth, which Robert Jordan loves. (We also learn at one point that Robert Jordan particularly loves their smell – further evidence.) So it's fitting that the story should begin and end with his heart pressed to the land he loves, and that he should die upon it. In their other occurrences, then, the pine needles would serve as momentary zoom-outs, to let us know that, whatever in particular might be happening (fighting, sex…). We're in Spain, drinking and fighting for that glorious Spanish-ness!
If you want to do something more specific with those pine-needles, be our guest. We're not really going there (because that seems a little too Symbolist for Hemingway's taste), but if you want to, the possibilities are endless.